Raym, without knowing exactly which washer you have the common F21 error is described in the service manual as
EXPLANATION AND RECOMMENDED PROCEDURE F21 LONG DRAIN
If the drain time exceeds 8 minutes the water valves turn off.
NOTES:After 4 minutes the “Sd” error will be displayed, then 4 minutes later the “F/21” error code will be displayed. Press PAUSE/CANCEL two times to clear the display.
Possible Causes/Procedure
1. Check the drain hose and make sure it is not plugged or kinked.
2. Unplug washer or disconnect power.
3. Check the electrical connections at the pump and make sure the pump is running.
4. Check the drain pump filter for foreign objects.
5. Plug in washer or reconnect power.
6. If the above does not correct the problem, go to step 7.
7. Unplug washer or disconnect power.
8. Replace the pump.
Sounds pretty extensive, especially the part about the pump. You do want to listen to make sure that the pump is not coming on. There is also the possibility that this error is caused by a failed water level switch. this would be cheaper and easier to repair. If you can give us the model of your washer, we can hopefully find out how to get your washer into diagnostic mode. That would allow us to either rule in or rule out the switch as the culprit.